1. Field of Invention
This invention relates to a telemetry system as applied particularly to seismographic exploration.
2. Related Prior Art
In seismic exploration, acoustic signals are injected into the earth. The acoustic signals radiate downwardly and are reflected from subsurface formations. The reflected acoustic signals then return to the earth's surface where they are detected by a plurality of seismic sensors or sensor groups. The sensors are deployed in groups along a cable with a spacing within the group of 10 to 20 feet or more. A plurality of groups may be distributed along a multiconductor cable system which typically may be as long as 10,000 feet. The detected, reflected acoustic signals are then transmitted to a central signal-recording system.
In accordance with conventional seismographic surveying practice the sensor groups are located along the cable at increasingly greater distances from the recording system. There is therefore, a nearest and farthest sensor group relative to the recording system. Commonly the plurality of seismic sensor groups transmit data to the recording system through a like number of physically separate transmission channels, typically two-wire cables. Whereas previously, standard practice called for use of about 50 data channels, modern exploration may require 500 channels of data. Because of the great expense of a large number of individual transmission channels, single-channel time-multiplexed telemetry systems (SCTMTS) have been proposed.
In such a (SCTMTS) system, the recording system polls each sensor group in sequence and identifies the individual sensor groups by an address unique to that sensor group. Alternately, various clocking schemes have been proposed wherein each sensor group has a unique response to one or more clocking signals emitted by the recording system. The previously suggested telemetry systems have a common characteristic in that each sensor group must in some physical way, be unique and distinguishable from the other sensor groups on a per-channel basis. Additionally in the case of a multichannel sensor group array, complex addressing systems must be used to change channel assignments, as various sensors are advanced in accordance with the so-called common depth point seismic exploration method. Thus, if each one of a large number of sensor groups must be identified uniquely, there is a likelihood of erroneous group identification. It would be very advantageous and would avoid confusion if all seismic sensor groups were identical and interchangeable. Such a system has been proposed in co-pending application Ser. No. 446,862.